Human macrophages and what appears to be a small population of thymic lymphocytes, react with unsaturated fatty acids (20:4 and C22:6). These lymphocytes will be defined in terms of known surface antigens. Rat alpha-fetoprotein derived from fetuses, maternal and hepatoma sera will be studied as to their variable contents of unsaturated fatty acids and their effects on the inhibition of phytohemagglutinin stimulated transformations of rat peripheral lymphocytes. The binding constants of both the human and rat source proteins with various fatty acids will be further explored. The sequence studies of human alpha-fetoprotein will be extended. The sequence of the WEIR lambda-chain will be determined since its dimer with the MCG L-chain can be crystallized in a form suitable for X-ray crystallographic studies. The sequences of these two lambda-chains will be related to the structural interactions in the crystalline hybrid protein and compared with results obtained for the MCG dimer. The influence of dimer formation between the WEIR and MCG L-chains on the immunological properties exhibited by the Bence-Jones forms of each protein will be studied and related to the chain interactions determined by X-ray diffraction. Work on defining the relationships of sequence to serologic factors (Mcg, Kern, Oz, St, 111) in various lambda chains will be extended.